The present invention has for its object an improved structure of multilayer circuits useful in the technological field of miniaturized electric components.
Multilayer circuits are normally constituted of a stack of thin layers, alternately conductive and insulating, which are deposited in superposition, one atop the other, by any suitable means, such as well known chemical, aqueous, or vacuum deposition processes and all others, all such procedures coming within the resin of the invention which seeks to correct defects which occur regardless of the technology utilized for the deposition of the layers.
These thin layers may, as required, be conformed during and with respect to the production of the stack to obtain desired volumetric electric circuit configurations. The term "thin layer" is intended here in its presently accepted meaning: layers of which the thicknesses are located from a few several hundreds of angstroms up to a small number of microns, less than 10.
A particular problem with respect to such multilayer circuits is that of short circuits which occur either during manufacture or in use. The elimination or minimization of short circuits is important, not only from the economics of manufacture, but also from the stand point of the reliability of the products.
In a general way, it is desirable that in multilayer circuits the conductive layers be relatively as thick as possible and the interposed insulating layers, relatively as thin as possible so that their electric efficiency is high. The short circuits appear more frequently as the insulating layers are made thinner, but it can also be stated that the frequency of short circuits increases with the thickness of the conductive layers. This is because, whatever the technique used for deposition, the state of the surface of a conductive layer degrades as the thickness increases. The growth of crystalline nature and the mesh or size of the crystals increases rapidly with thickness primarily under the effect of the macles or twins which are produced and the accumulation of defects in the microscopic range causing most often small crystalline growths. However, a very thin layer reproduces accurately the state of the surface of the substrate on which it is deposited. It follows that when a very thin insulating layer is formed on a relatively thick conductive layer, whose surface condition is poor for the reason above discussed, the very thin insulating layer will reproduce all of the defects of the surface of the conductive layer and will prevent variations of thickness leading readily to the existence of microporosities and the appearance of short circuits.
These risks of production of short circuits during manufacture therefore introduce a limitation on the ratio of the thicknesses of the condutive and insulating layers in multilayer circuits which is by no means negligible.
Further, multilayer circuits must often support during use of equipment in which they are incorporated large elevations of temperature which can for example reach 450.degree. C. or thereabout. However, the conductive materials currently utilized in these circuits, because they have a low resistivity favorable to the flow of electric current, are, usually, the four metals of the group of copper, aluminum, silver, gold and their alloys. Each of these standard metals has a coefficient of its thermal expansion which is relatively large, that for copper for example being 14 10.sup.-6 /.degree. C., and crystallize easily when the temperature increases, with grains which enlarge quickly. The lattice of the crystalline structure is therefore under considerable stresses entailing or producing the formation of fissures. Thus, a metal-insulator-metal structure can develop a short circuit when submitted to an elevation of temperature which modifies the regular crystalline structure of the metal.
The object of the invention is to eliminate the above-described difficulties and, particularly, to provide multilayer circuits with any desirble high ratio between the thicknesses of the conducting and insulating layers. The problem of the elevations of temperatures with respect to the insulating layers can be ignored since the materials usual for these insulating layers have high thermal stability at the temperature to be considered for the efficiency of the circuits.